The mechanics of human injury is a complex process whereby external forces are transmitted to the human musculo-skeletal system, the system responds in the manner of displacements and rotations, stresses are induced in the tissues, and under some conditions the tissues are disrupted. When the forces are not constant, prediction of the likelihood of injury and the severity of the forces is difficult. This proposal presents a research program investigating the mechanics of lower extremity injury, specifically injuries to the knee, tibia and ankle that typically occur in snow skiing. Snow skiing is studied because the lower extremity injury rate is high, because the forces transmitted to the foot can be measured, because response of the lower extremity can be measured, and because safety devices can reduce the high rate of injuries. Specialized laboratory and field test equipment have been developed to measure and analyze the forces between the boot and the ski, the rotations occurring at the ankle, knee, and pelvis, and the integrated EMG from muscle groups during skiing and especially during falling when severe loading occurs and the likelihood of injury increases. The field measurements identify the injury environment and the laboratory experiments clarify how the lower extremity responds to dynamic loading. This is the "identification" problem for the lower extremity. The true severity of the typical skiing environment and the contributions of the musculature to influence the likehood of injury will be clarified. The common misconception that the forces of skiing are small compared to typical tibia fracture strength and knee ligament strength will be corrected. The erroneous concept is widespread, extending to standards organizations, the industry and the public alike. The error has major impact on the design of safety devices, on the evaluation and acceptance of safety devices, and on the training and instruction given the public. Efforts are directed to development of meaningful standards of safety.